Denis Douillet

Hartmann wave-front measurement at 13.4 nm with λ EUV /120 accuracy

We report, for the first time to our knowledge, experimental demonstration of wave-front analysis via the Hartmann technique in the extreme ultraviolet range. The reference wave front needed to calibrate the sensor was generated by spatially filtering a focused undulator beam with 1.7- and 0.6-microm-diameter pinholes. To fully characterize the sensor, accuracy and sensitivity measurements were performed. The incident beams wavelength was varied from 7 to 25 nm. Measurements of accuracy better than lambdaEUV/120 (0.11 nm) were obtained at lambdaEUV = 13.4 nm. The aberrations introduced by an additional thin mirror, as well as wave front of the spatially unfiltered incident beam, were also measured.

Towards ultrahigh-contrast ultraintense laser pulses--complete characterization of a double plasma-mirror pulse cleaner

The effects of small amounts of energy delivered at times before the peak intensity of ultrahigh-intensity ultrafast-laser pulses have been a major obstacle to the goal of studying the interaction of ultraintense light with solids for more than two decades now. We describe implementation of a practical double-plasma-mirror pulse cleaner, built into a f=10m null telescope and added as a standard beamline feature of a 100 TW laser system for ultraintense laser-matter interaction. Our measurements allow us to infer a pulse-height contrast of 5×1011—the highest contrast generated to date—while preserving ∼50% of the laser intensity and maintaining excellent focusability of the delivered beam. We present a complete optical characterization, comparing empirical results and numerical modeling of a double-plasma-mirror system.

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Multicolour above-threshold ionization of helium: quantum interference effects in angular distributions

Energy- and angle-resolved photoionization spectra of He irradiated by linearly polarized intense 810 nm laser radiation and several of its XUV odd harmonics are investigated. The angular distribution of the odd-order peaks, produced by single-photon ionization by one harmonic, is, surprisingly, broadened by the IR field. The even-order ones, due to two-colour, two-photon ionization, show at 90° lobes which depend on the relative IR-XUV phase. Application to the characterization of attosecond pulses is suggested.

X-ray beam metrology and X-ray optic alignment by Hartmann wavefront sensing

In 2002, first experiments at the Advanced Light Source (ALS) at Berkeley, allowed us to test a first prototype of EUV Hartmann wave-front sensor. Wave-front measurements were performed over a wide wavelength range from 7 to 25 nm. Accuracy of the sensor was proved to be better than λEUV/120 rms (λEUV = 13.4 nm, about 0.1 nm accuracy) with sensitivity exceeding λEUV/600 rms, demonstrating the high metrological performances of this system. At the Swiss Light Source (SLS), we succeeded recently in the automatic alignment of a synchrotron beamline by Hartmann technique. Experiments were performed, in the hard X-ray range (E = 3 keV, λ = 0.414 nm), using a 4-actuators Kirkpatrick-Baez (KB) active optic. An imaging system of the KB focal spot and a hard X-ray Hartmann wave-front sensor were used alternatively to control the KB. The imaging system used a genetic algorithm to achieve the highest energy in the smallest spot size, while the wave-front sensor used the KB influence functions to achieve the smallest phase distortions in the incoming beam. The corrected beam achieved with help of the imaging system was used to calibrate the wave-front sensor. With both closed loops, we focused the beam into a 6.8x9 μm2 FWHM focal spot. These results are limited by the optical quality of the imaging system.

The Extreme Light Infrastructure Project ELI and Its Prototype APOLLON/ILE: “The Associated Laser Bottlenecks”

We will present the ELI Project consisting in a 200PW laser based high intensity physics users infrastructure. A French single beamline laser prototype called ILE/APOLLON is presently under construction. Associated technological bottlenecks will be described.

Full characterization of a laser-produced keV x-ray betatron source

This paper presents the complete characterization of a kilo-electron-volt laser-based x-ray source. The main parameters of the electron motion (amplitude of oscillations and initial energy) in the laser wakefield have been investigated using three independent methods relying on spectral and spatial properties of this betatron x-ray source. First we will show studies on the spectral correlation between electrons and x-rays that is analyzed using a numerical code to calculate the expected photon spectra from the experimentally measured electron spectra. High-resolution x-ray spectrometers have been used to characterize the x-ray spectra within 0.8–3 keV and to show that the betatron oscillations lie within 1 µm. Then we observed Fresnel edge diffraction of the x-ray beam. The observed diffraction at the center energy of 4 keV agrees with the Gaussian incoherent source profile of full width half maximum <5 µm, meaning that the amplitude of the betatron oscillations is less than 2.5 µm. Finally, by measuring the far field spatial profile of the radiation, we have been able to characterize the electrons trajectories inside the plasma accelerator structure with a resolution better than 0.5 µm.

X ray Wavefront Hartmann Sensor

We report an experimental demonstration of wavefront analysis via the Hartmann technique in the EUV. The reference wave front needed to calibrate the sensor was generated by spatially filtering a focused undulator beam with 1.7 (0.6) μm pinhole. To fully characterize the sensor, accuracy and sensitivity measurements were performed. The incident beam’s wavelength was varied between 7 and 25 nm. Measurements of accuracy better than λEUV=120 (0.11 nm) were obtained λEUV=13.4 nm. The wavefront of the spatially unfiltered incident beam was also measured.

Effect of the Laser Wave Front in a Laser-Plasma Accelerator

A high-repetition rate electron source is generated by tightly focusing kHz, few-mJ laser pulses into an underdense plasma. This high-intensity laser-plasma interaction leads to stable electron beams over several hours but with strikingly complex transverse distributions even for good quality laser focal spots. We find that the electron beam distribution is sensitive to the laser wave front via the laser midfield distribution rather than the laser focal spot itself. We are able to measure the laser wave front around the focus and include it in realistic particle-in-cell simulations demonstrating the role of the laser wave front on the acceleration of electrons. Distortions of the laser wave front cause spatial inhomogeneities in the midfield laser intensity and, consequently, the laser pulse drives an inhomogeneous transverse wakefield whose focusing and defocusing properties affect the electron distribution. These findings explain the experimental results and suggest the possibility of controlling the electron spatial distribution in laser-plasma accelerators by tailoring the laser wave front.

Anomalous high-order harmonic generation

We present experimental harmonic spectra showing anomalously high orders generated at low laser intensities (around 2 × 1013 W cm−2) in argon, krypton and xenon. These high orders cannot be explained with the usual three step model that predicts the existence of a sharp cut-off after the region of constant efficiency called the plateau: the observed orders are far above the cut-off limit inferred from the laser intensity in the medium. The systematic study of these spectra are presented, and some possible theoretical models are explored.